Stem cells and tissue regeneration have great therapeutic promise for treatment of a wide variety of medical conditions. For example and relevant to the current application, extremity injuries compromise 50-60% of all combat casualties observed in Iraqi/Afghanistan War Veterans resulting in large soft tissue defects (muscle and skin) and high amputation rates. Skeletal muscle, while being the tissue most vulnerable to ischemic damage in the extremities, also has an amazing potential to regenerate due to satellite cells. Satellite cells are stem cells that reside in skeletal muscle and with muscle injury, can proliferate and fuse together or with damaged muscle fibers to regenerate muscle; macrophages are critical to this process. Multiple types of macrophages participate in the regenerative process; classically activated (M1), wound healing (M2a) and regulatory (M2c) macrophage subsets are present in skin wounds and regenerating skeletal muscle; promoting inflammation, tissue repair and resolution of inflammation, respectively. However, the regulation of macrophage specialization (polarization) by microRNAs (miRNAs), and the consequential effects of macrophage subsets on muscle regeneration have not been elucidated. MiRNAs are small, noncoding RNAs that inhibit gene expression, thereby regulating many processes, including immune cell differentiation. A better understanding of the mechanisms of skeletal muscle regeneration and skin wound healing, and the influence of macrophage polarization on these events, could lead to new and adjunct therapies for limb salvage. Our long-term goal is to define the influence of inflammation, including the chemokine system, in angiogenesis, wound healing and skeletal muscle regeneration. We have extensively studied the importance of the CC Chemokine Receptor 2 (CCR2) in muscle regeneration. CCR2 is crucial for monocyte/macrophage recruitment and differentiation. Following muscle injury, CCR2-/- mice have impairments in macrophage recruitment, angiogenesis and muscle regeneration compared to wild type (WT) mice. Importantly, macrophage recruitment and muscle regeneration defects can be reversed in CCR2-/- mice by providing WT bone marrow, thus, CCR2 expression on bone marrow-derived cells is critical for normal muscle regeneration and macrophages are the likely BM-derived cell that mediates this outcome. The following 3 specific aims will be tested: 1) Define the in vitro regulation of macrophage polarization by selected miRNAs (miR-21 and -147). 2) Establish the mRNA targets of selected miRNAs (miR-21 and -147) during macrophage polarization and 3) Determine the biological effects of individual miRNAs on in vivo macrophage subsets and coordinated biological events during wound healing. By modulating macrophage polarization via increased or decreased miRNA expression, we seek to improve wound healing and decrease the adverse affects of acute and chronic inflammation present in many diseases that affect the Veteran population. The proposed studies are innovative because they will help define the contribution of miRNAs to macrophage polarization and subsequent muscle regeneration and wound healing. The combination of in vitro and in vivo studies will collectively identify miRNAs important for macrophage polarization, determine the mRNA targets, and assess any effects on muscle regeneration and wound healing. Given the availability of locked-nucleic-acid-modified oligonucleotide (LNA-antimiR), RNA oligonucleotides complementary to specific miRNAs that can be used in animals to decrease miRNA expression, a new therapeutic class of agents could become available for humans. The significance of this research is that a better understanding of the mechanisms of skeletal muscle regeneration and wound healing could lead to the design of novel primary or adjuvant treatments for improved limb salvage using miRNA- altering compounds. PUBLIC HEALTH RELEVANCE: Leg and arm wounds, with large muscle defects and high amputation rates, are common in trauma victims and especially in injured soldiers from the Iraqi/Afghanistan war; new treatments to replace the missing muscle are needed to decrease amputation rates and improve limb function. Our research studies the complex relationships between the multiple cells that are needed to make new muscle, including bone marrow cells. Macrophages are white blood cells that travel from the bone marrow to areas of injury to remove dead tissue and assist in the healing process. Different types of macrophages assist in wound healing; being able to change the type of macrophage present in a wound could result in improved muscle regeneration. A better understanding of how macrophages contribute to recovery from injury could lead to new therapies, including tissue engineering strategies, to help patients recover from these devastating injuries.